1. Field of the Invention
The present invention relates to a control apparatus for controlling operation of a reversible electric motor while ensuring a fail-safe function for the operation of the motor. Further, the invention is concerned with a motor-driven power steering system of a motor vehicle or automobile which is adapted for assisting a driver in manipulating a steering wheel or handle by making use of an assist torque generated by a reversible motor which is mechanically coupled to the steering system. In particular, the present invention is concerned with an apparatus for controlling operation of the motor-driven power steering system of the type mentioned above, which apparatus is imparted with a capability of detecting not only a fault such as a ground fault of the reversible motor but also the occurrence of an abnormality in the control circuit therefor to protect the motor against adverse influence of such fault and abnormality. Hereinafter, the motor-driven power steering system and the control apparatus mentioned above will also be referred to collectively as the motor-driven power steering control system.
2. Description of the Related Art
In a motor-driven (or electrically driven) power steering control system for a motor vehicle, a steering torque applied to a steering wheel by a driver is detected by a torque sensor to thereby allow a reversible electric motor of the power steering system to generate an assist torque of such magnitude which is substantially proportional to the detected steering torque while taking into account the speed of the motor vehicle (hereinafter referred to as the vehicle speed), wherein the assist torque thus generated is applied to a steering shaft assembly for the purpose of aiding or facilitating the driver in manipulating the steering wheel. In the motor vehicle equipped with such motor-driven power steering control system, there may arise a very unfavorable situation for driving the motor vehicle when a ground fault occurs in the reversible motor of the steering system in the electrically energized state, because then a very large current will flow through the motor, whereby an assist torque of a magnitude which far exceeds what the driver expects will be generated. For this reason, the motor-driven power steering control system is generally equipped with a fail-safe means for detecting the occurrence of the ground fault to thereby interrupt an electric power supply to a motor driver circuit which is employed for driving the reversible motor of the power steering system.
As the motor-driven power steering control systems of the above type known heretofore, there may be mentioned those described in, for example, Japanese Unexamined Patent Application Publication No. 251596/1992 (JP-A-H4-251596) and Japanese Utility Model Application No. 20976/1993 (JP-UA-H5-20976). For better understanding of the subject matter of the present invention, description will first be made in some detail of these hitherto known control systems.
FIG. 8 is a block diagram showing generally a structure of a motor-driven power steering control system disclosed in JP-A-H4-251596. As can be seen in the figure, this prior art motor-driven power steering control system is comprised of a central processing unit 100, a drive control circuit 101, a gate driver circuit 102 for generating PWM (Pulse Width Modulated) signals, and a motor control circuit 103 constituted by controlled rectifier elements such as field effect transistors (FETs) connected in the form of a H-bridge circuit and serving as a switching means for controlling an electric motor 104 in the forward and backward rotating directions, wherein the switching operation of the motor control circuit 103 is controlled by the central processing unit 100 via the gate driver circuit 102 for thereby allowing the assist torque generating motor 104 to be rotated in the forward direction or in the backward (reverse) direction. The drive control circuit 101 and the gate driver circuit 102 cooperate to constitute a motor driving means for driving controllably the motor control bridge circuit 103.
Further, the motor-driven power steering control system is provided with a fail-safe means which is constituted by a shunt resistor 107, a motor current detection circuit 108 and an overcurrent detection circuit 109.
More specifically, a relay 105 is connected to a power source 106 for supplying an electric power to the motor control circuit 103. The shunt resistor 107 is connected between the relay 105 and the motor control bridge circuit 103. The motor current detection circuit 108 is connected across the shunt resistor 107 for detecting a current flowing to the motor 104 on the basis of a voltage appearing across the shunt resistor 107. The overcurrent detection circuit 109 is connected to the motor current detection circuit 108 for detecting the occurrence of an overcurrent state on the basis of the current detection level outputted from the overcurrent detection circuit 109.
In operation, when a large current flows through the assist torque generating motor 104 upon the occurrence of a ground fault, the large current is detected by the motor current detection circuit 108. On the other hand, the overcurrent detection circuit 109 detects the overcurrent state on the basis of the output of the motor current detection circuit 108 and supplies corresponding information to the central processing unit 100 which responds thereto by turning off the relay 105, whereby the power supply to the motor control bridge circuit 103 from the power source 106 is interrupted. Further, operations of the drive control circuit 101 and the gate driver circuit 102 are inhibited in response to the overcurrent state detection signal outputted from the circuit 109.
As is apparent from the above, the reversible motor control system disclosed in JP-A-H4-251596 is designed to detect only the ground fault, and is incapable of detecting other abnormalities which may occur, for example, in the bridge circuit 103 or the control circuits 101 and 102.
FIG. 9 is a block diagram showing generally a structure of the motor-driven power steering control system disclosed in JP-AU-H5-20976. Referring to the figure, this prior art motor-driven power steering control system is comprised of a control unit 200 and a motor control bridge circuit 201 implemented in the form of H-bridge circuit including gate-controlled semiconductor switch elements such as FETs, wherein change-over of the direction in which an assist torque generating reversible motor 202 is to be driven as well as magnitude of torque generated thereby is controlled via the motor control bridge circuit 201 with PWM signals generated by the control unit 200.
Further, first and second current detection circuits 203 and 204 are connected to the motor 202 at upstream and downstream sides thereof, respectively. A fail-safe relay circuit 206 is interposed between the motor control bridge circuit 201 and a power source 205. The current detection circuits 203 and 204 cooperate with the fail-safe circuit 206 to constitute a fail-safe means.
In operation, the current flowing to the reversible motor 202 is detected by the first current detection circuit 203, while a current drained from the reversible motor 202 is detected by the second current detection circuit 204, wherein difference between the outputs of the first and second current detection circuits 203 and 204 is determined by the control unit 200. When the difference mentioned above exceeds a predetermined current value, it is then decided that a ground fault takes place. In that case, the power supply to the motor control bridge circuit 201 from the power source 205 is interrupted by turning off a relay switch incorporated in the fail-safe relay circuit 206.
In the motor-driven power steering control system disclosed in JP-A-H4-251596, the fail-safe means which is comprised of only one motor current detecting circuit 108 for detecting the motor current on the basis of the voltage appearing across the shunt resistor 107 is limited in respect to its abnormality detecting capability. More specifically, with the motor current detecting circuit 108, it is certainly possible to detect a ground fault of the reversible motor 104. However, solely with this circuit 108, it is difficult or impossible to detect the occurrence of an abnormality in the motor driving means constituted by the drive control circuit 101 and the gate driver circuit 102.
In contrast, in the case of the motor-driven power steering control system disclosed in JP-A-H5-20976, there are provided two current detection circuits, i.e., the first and second current detection circuits 203 and 204. Accordingly, the problem mentioned above can certainly be avoided. However, the provision of the two current detection circuits 203 and 204 as indispensable components necessarily leads to high cost and increased scale of the motor-driven power steering control system, giving rise to another problem.
It is further noted that in both of the motor-driven power steering control systems disclosed in JP-A-H4-251596 and JP-UA-H5-20976, validation of the fail-safe function is based on current detection. In other words, the abnormality mentioned above can be detected only in the state where the reversible motor 104 or 202 is actually being driven. Consequently, such abnormality that no PWM signal is outputted due to failure in the gate driver circuit 102 and/or the control unit 200 can not be detected, to another disadvantage.